A wind turbine in operation will not always experience wind perpendicular to a rotor plane. When the rotor plane (which is also referred to as heading) of a wind turbine is not perpendicular to the wind, the efficiency will decrease. Therefore, actual wind turbines comprise a yaw system designed to automatically adjust their heading, like, e.g., rotating the rotor plane perpendicular to the incoming wind or to maintain an angle relative to the wind to maximize the surface area of the turbine rotor.
Usually, the yaw system is part of a nacelle, which may be involved in a yawing movement, i.e. being rotatable mounted on top of a tower via at least one yaw bearing. A rotor is attached to an upwind side of the nacelle. The rotor is coupled via a drive train to a generator housed inside the nacelle. The rotor includes a central rotor hub and a plurality of blades mounted to and extending radially from the rotor hub defining the rotor plane.
It is important for wind power plant operators to know an actual position or direction of the rotor plane or heading of the respective wind turbine, the plane or heading being correlated with an actual position or direction of the nacelle. The actual direction of the nacelle is also referred to as a yaw direction or a yaw position or, in relation to a predefined direction (e.g. a cardinal direction), as a yaw angle. Alternatively the yaw angle may be defined as the direction of the nacelle in relation of the direction of the incoming wind.
FIG. 1 shows in a schematically top view an exemplary scenario of a wind turbine 100 in relation to the well known cardinal points or compass points which are indicated as a compass rose in the background of FIG. 1. A rotor hub 120 including a plurality of blades 130 defining a rotor plane 140 is mounted at the upwind side of a nacelle 110. According to the scenario of FIG. 1, an actual yaw direction 150 (which is also referred to as “compass heading”) of the wind turbine 100, i.e. the actual direction of the nacelle 110 points towards the cardinal direction “North East” or “NE”. As exemplarily shown in FIG. 1, an absolute yaw angle “θYawAngle” is referencing the actual yaw direction 150 of the wind turbine in relation towards the cardinal direction “North” or “N”. The absolute yaw angle θYawAngle is indicated by an arrow 160, wherein θYawAngle=45°.
Information concerning the yaw direction is a common used basis for analyzing data concerning a wind turbine or performing sector management control like, e.g.,                site wind mapping and historical data collection on wind patterns,        limiting wind turbine noise by avoiding operation in wind directions where noise generation is excessive,        automatic curtailment and regulation of a wind turbine at yaw angles where significant wind turbulence might be present,        prevention of shadow flicker/light pollution for neighboring residents or businesses at certain times of day and yaw angles,        remote manual control of a wind turbine yaw position,        efficiency testing and wind turbine power curve validation, or        safe positioning of the rotor during ice conditions when service teams are approaching.        
In order to determine, e.g., an absolute yaw angle, a wind turbine may be equipped with a yaw encoder, measuring the relative yaw direction in relation to a stationary object like, e.g., a tower being secured to a foundation at ground level. The yaw encoder is typically calibrated by determining a reference yaw direction or reference yaw angle after finalization of the wind turbine installation.
In some scenarios the initial calibration of the yaw angle is incorrect or less accurate due to applying a rough estimate or rule of thumb to determine a cardinal direction as a basis or reference for the yaw angle calibration.
A further possible reason for an inaccurate yaw angle calibration is a wind turbine installation based on a design including powerful permanent magnets, eliminating the possibility of applying magnetic compasses to determine the yaw direction or yaw angle. A magnetic compass, as a further general disadvantage, comprises inaccurateness per se, in particular at installations located at high geographic latitudes.
Alternatively, compasses based on GPS (Global Positioning System) or other satellite-based positioning systems have been applied to determine the reference yaw direction of the wind turbine.
[EP 2 599 993 A1] refers to a method to determine the yaw angle of a component of a wind turbine wherein at least one receiver of an automated and autonomous positioning system is used to generate position-data of the receiver. The receiver is arranged at a wind turbine location being subjected to a yawing movement.
However, applying such kind of automated and autonomous positioning systems for calibration issues is restricted due to high costs and limited accuracy.